There are multiple ways of converting from one format to another, just
like there are multiple ways of converting letters to numbers. For example,
you might map A to 1, B to 2, C to 3, etc. Or you might map A to 0, B to 1,
C to 2, etc. Before worrying about speed, you should probably think about
what values map onto what other values first.

   There are multiple ways of doing this. The simplest would be to
concatenate the sequences of bytes together, but then you lose the
information of where each sequence starts and end. To get around this, you
might want to additionally encode the length of each sequence, for example,
or have some sort of special "end of sequence" marker.

   - Oliver

Why do you need to convert them?

What should that byte array satisfy?
Obviosuly, you could serialize the Calendar into a byte array, but
that's hardly fast. On the other hand, it is probably the simplest
to implement when you want to serialize several different objects
into one byte sequence.

Or you could do
 long time = calendar.getTimeInMillis();
 byte[] timeBytes = new byte[]{
    (byte) (time >> 56),
    (byte) ((time >> 48) & 255),
    (byte) ((time >> 40) & 255),
    (byte) ((time >> 32) & 255),
    (byte) ((time >> 24) & 255),
    (byte) ((time >> 16) & 255),
    (byte) ((time >> 8) & 255),
    (byte) (time & 255),
 };
or something similar.

Perhaps
 string.getBytes() // or better yet, pick an encoding

Again, it depends on what you need to use it for.

java.security.cert.Certificate, javax.security.cert.Certificate,
or another specialization of java.security.Certificate?

Probably either getEncoded() or encode(ByteStream).

The PrivateKey interface also has a getEncoded() method.

byte[] bigArray = new byte[array1.length + array2.length];
System.arraycopy(array1, 0, bigArray, 0, array1.length);
System.arraycopy(array2, 0, bigArray, array1.length, array2.length);

Obviously this is not storing enough information to split the big
array into smaller ones.

I think what is worrying me is your request for a "fastest" way,
since that's probably less understandable and maintainable than
just the simplest way.

/L

1- The question is whether you want to deserialize it, too. Moreover, do you
want the complete information restored or just a timestamp.

2- Fastest? String.getBytes() is a pretty good candidate. Perhaps you could do
better if you have an additional information regarding your strings' structure.

3- and 4- sound suspiciously like you want to store them somewhere. There are
better ways to do that than simply "serializing" a object.

As for 5-, you have to allocate an array as big as to accommodate both arrays
and then use System.arraycopy twice. An alternative is to use ByteBuffer, two
.put()s and an .array(), which basically does the same thing behind the scenes.